Method for Fabricating a Vertical Light-Emitting Diode with High Brightness

ABSTRACT

A method for fabricating a vertical light-emitting diode comprises forming a stack including a plurality of epitaxial layers on a patterned first substrate, placing a second substrate on the stack, removing the first substrate to expose the first surface, planarizing a first surface of the stack that was in contact with the patterned first substrate and has a pattern corresponding to a pattern provided on the first substrate to form a planarized second surface, and forming a first electrode in contact with a side of the second substrate that is opposite to the stack, and a second electrode in contact with the second surface of the stack. A roughening step can be performed to form uneven surface portions on a region of the second surface for improving light emission through the second surface of the stack.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Taiwan Patent Application No.099134805, filed on Oct. 12, 2010.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods for fabricating alight-emitting diode, and more particularly to a method for fabricatinga vertical light-emitting diode that has high brightness.

2. Description of the Related Art

Light-emitting diodes (LED) are widely used in lighting devices anddisplay devices. In a conventional process for fabricating alight-emitting diode, the multilayered light-emitting structure isepitaxially grown on a sapphire substrate. Owing to the low electricalconduction and thermal dissipation of the sapphire substrate, twoelectrodes may be formed on the same side of the light-emitting diode toform a horizontal type light-emitting diode. However, the laterallight-emitting diode has certain disadvantages, including currentcrowding effect and high forward voltage. Accordingly, the horizontaltype light-emitting diode may have poor efficiency and output power.

In order to overcome the disadvantages of low electrical conduction andthermal dissipation, one approach proposes the structure of a verticallight-emitting diode. In a vertical type light-emitting diode, the twoelectrodes are disposed on the top of light-emitting structure and theback side of the substrate, respectively. To fabricate the vertical typelight-emitting diode, a buffer layer is first formed on a sapphiresubstrate. Nitride semiconductor compounds can be then grown on thebuffer layer to form the light-emitting structure. A conductivesubstrate (such as a metal substrate) then can be placed on thelight-emitting structure, followed with the removal of the sapphiresubstrate. Electrodes then can be formed on the back side of theconductive substrate and the top of the light-emitting structure,respectively.

Certain approaches have been proposed to improve the process offabricating a vertical light-emitting diode, such as described in TaiwanPat. No. 1294700, Taiwan patent No. 1293813, Taiwan applicationpublication No. TW201010127, Taiwan patent No. 1304660, and Taiwanpatent No. 1315915, the disclosure of which is incorporated herein byreference. Taiwan patent No. 1294700 relates to a light-emittingstructure in which a second clad layer with an uneven thickness is usedto adjust the electrical resistance to equilibrium, so that the lightcan be emitted uniformly. Taiwan patent No. 1293813 discloses using anadhering reflection layer to improve the binding between thelight-emitting structure and the supporting substrate, and enhanceemission efficiency. Taiwan application publication No. TW201010127discloses binding the supporting substrate with a conductive adhesive toprevent epitaxial fracture that may be induced by the removal of thesapphire substrate via laser lift-off. The disclosure of Taiwan patentNo. 1304660 relates to the use of chip-bonding in the manufacture of avertical light-emitting diode. Taiwan patent No. 1315915 discloses thata fabrication method in which a second semiconductor layer is formed ona second substrate having an indentation structure, and an electrode isthen formed on a corresponding indentation structure of thesemiconductor layer.

In the aforementioned manufacture methods, point defect or line defectof the epitaxial layer may easily occur because the lattice constant andthe coefficient of thermal expansion of the nitride compound differ fromthose of the sapphire substrate. Such defects may adversely affect thecharacteristics of the light-emitting diode, such as reduced brightness,and larger input current to achieve similar output efficiency.Therefore, there is a need for a method fabricating a verticallight-emitting diode that can address at least the foregoing issues.

SUMMARY

The present application describes a method for fabricating a verticallight-emitting diode. In some embodiments, the method comprises forminga stack including a plurality of epitaxial layers on a patterned firstsubstrate, placing a second substrate on the stack, removing the firstsubstrate to expose the first surface, planarizing a first surface ofthe stack that was in contact with the patterned first substrate and hasa pattern corresponding to a pattern provided on the first substrate toform a planarized second surface, and forming a first electrode incontact with a side of the second substrate that is opposite to thestack, and a second electrode in contact with the second surface of thestack. A roughening step can be performed to form uneven patterns on aportion of the second surface c for improving light emission through thesecond surface of the stack.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1F are schematic views illustrating intermediary stages in thefabrication of a vertical light-emitting diode;

FIG. 2 is a schematic view illustrating another embodiment of a methodfor fabricating a vertical light-emitting diode; and

FIG. 3 illustrates various geometrical shapes of the pattern provided ona sapphire substrate.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present application describes a method for fabricating a verticallight-emitting diode comprised of a stack of multiple epitaxial layers.The stack of the epitaxial layers can be formed on a patterned surfaceof a sapphire substrate. After the substrate is removed, planarizationcan be applied on the surface of the stack that was in contact with thepatterned surface of the sapphire substrate, and an electrode layer thencan be formed on the planarized surface of the stack. A roughening stepcan also be performed to form uneven patterns on a portion of the secondsurface for improving light emission through the second surface of thestack. The method described herein can be applied to fabricate variousvertical light-emitting diodes, especially vertical light-emittingdiodes with high brightness.

“Group III nitride” as employed herein can refer to a compound thatcontains nitrogen (N) and a chemical element belonging to the group IIIof the periodic table such as aluminum (Al), gallium (Ga), indium (In)and the like, as well as any compound thereof (such as AlGaN, AlInGaN).In one embodiment, Al_(x)In_(y)Ga_(1-x-y)N (0≦x≦1, 0≦y≦1) or oxidesemiconductor material can be incorporated in the multilayer stack ofthe light-emitting diode.

FIGS. 1A through 1F are schematic views illustrating intermediary stagesin the fabrication of a vertical light-emitting diode. Referring to FIG.1A, a sapphire substrate 21 can be first provided. The sapphiresubstrate 21 can have a surface that is treated to form a patternthereon. A buffer layer 22 made of a group III nitride compound (such asGaN) can be formed on the patterned surface of the sapphire substrate21. The formed buffer layer 22 can have a surface I in contact with thesapphire substrate 21.

The patterned surface of the sapphire substrate 21 on which is formedthe buffer layer 22 can be provided with a pattern of variousgeometrical shapes including, without limitation, pyramid shapes, posts,half lenticular or concave or convex shapes, conical shapes and thelike. These patterns can be formed as projections including taperedprofiles, non-tapered profiles or a combination thereof. FIG. 3illustrates different examples of patterns including, withoutlimitation, smooth half-oval (as shown in (a) of FIG. 3), pyramid shapes(as shown in (b) of FIG. 3), cones (as shown in (c) of FIG. 3),truncated cones (as shown in (e) of FIG. 3), truncated pyramids (asshown in (f) of FIG. 3) and the like. Non-tapered patterns can includepillar shapes, such as shown in (d) of FIG. 3. It will be appreciatedthat the patterned surface of the sapphire substrate 21 is not limitedto the foregoing examples of shapes, and other geometry may beapplicable.

The patterned surface of the sapphire substrate 21 can significantlyreduce a defect density in the epitaxial layers formed thereon, andenhance the properties of the light-emitting diode. The protrudingshapes patterned from the surface of the sapphire substrate 21 canresult in the first surface I of the buffer layer 22 being formed withcorresponding recessed and protruding shapes. The buffer layer 22 can bean epitaxial layer formed by metal-organic chemical vapor deposition(MOCVD) or molecular beam epitaxy (MBE).

Next referring to FIG. 1B, a multilayered light-emitting structure 23comprised of multiple epitaxial layers can be formed on the buffer layer22. The multilayered light-emitting structure 23 can be formed by MOCVDor MBE, and include group III nitride semiconductor materials and/oroxide semiconductor materials. In one embodiment, the multilayeredlight-emitting structure 23 can include a n-GaN layer 23 a, alight-emitting layer 23 b and a p-GaN layer 23 c respectively stacked onthe buffer layer 22. In one embodiment, the light-emitting layer 23 bcan be formed as a structure of multiple quantum well (MQW) layers.

Next referring to FIG. 1C, a bonding layer 24 can be formed on the p-GaNlayer 23 c, and a conductive substrate 25 then can be attached onto thebonding metal layer 24. The bonding layer 24 can be made of a metallicmaterial. The above buffer layer 22, multilayered light-emittingstructure 23, bonding layer 24 and conductive substrate can begrown/assembled in accordance with any known methods.

Next referring to FIG. 1D, the sapphire substrate 21 can be removed fromthe first surface I. Methods applied for removing the sapphire substrate21 can include laser lift-off, etching or polishing. In one embodiment,laser lift-off can be preferably used. KrF excimer laser with awavelength of 248 nm can be applied on the side of the sapphiresubstrate 21 so that dissociation at the portion of the buffer layer 22near the sapphire substrate 21 can occur. Then, heating can be appliedat a temperature between about 30-40° C. to peel the sapphire substrate21. As a result, the first surface I can be exposed with a pattern ofshapes corresponding to the patterned surface of the sapphire substrate21.

The uneven profile of the first surface I may cause total internalreflection (TIR), which can result in reduced light extraction andincreased heating in the light-emitting diode. A surface treatment canbe conducted to planarize the first surface I after removal of thesapphire substrate 21.

Referring to FIGS. 1D and 1E, a planarization step can be applied on thestack of epitaxial layers. Selective wet etching or polishing can beconducted to planarize the first surface I. In one embodiment, the stackof epitaxial layers can be cleaned and immersed in an etching agent at atemperature between about 20-200° C. Through planarization, the firstsurface I can be converted into a planarized surface II. Subsequently,the etching agent can be removed, and the stack of epitaxial layers canbe rinsed in an organic solution (such as acetone, methanol, isopropanolor ethanol).

In the aforementioned planarization step, the used etching agent canselectively etch a nitride semiconductor material. In some embodiments,the etching agent can be an acid solution selected from H₂SO₄, H₃PO₄,HNO₃, HNO₂, H₃PO₃, HCl, CH₃COOH, H₂CO₃, H₂BO₃, HCOOH, HIO₃, H₂C₂O₄, HF,H₂S, H₂SO₃, HSO₃F, RSO₃F (R═C_(n)H_(2n+1)), or any mixture thereof. Inother embodiments, the etching agent can be an alkaline solutionselected from NaOH, KOH, Ca(OH)₂, TMAH, NH₄OH, Na₂CO₃, NaHCO₃, K₂CO₃,Ba(OH)₂, or any mixture thereof. The etching agent (acidic solution oralkaline solution) can exhibit a high etching rate with respect to anuneven or unsmooth surface of a nitride semiconductor material, andalmost no etching or extremely low etching rate with respect to a flatsurface of a nitride semiconductor material. Accordingly, the coarseprofile of the first surface I can be effectively planarized with theetching agent. It is worth noting that because the buffer layer 22 isrelatively thin, it may be completely removed after planarization.Accordingly, the planarized surface II can be defined as a surface ofthe n-GaN layer 23 a.

Next referring to FIG. 1F, a first electrode 26 can be formed on anexposed side of the conductive substrate 25 opposite to the side of theplanarized surface II, and a second electrode 27 can be formed on theplanarized surface II of the multilayered light-emitting structure 23(e.g., on the top of the n-GaN layer 23 a). The first electrode 26 andthe second electrode 27 can be formed in contact with the conductivesubstrate 25 and the n-GaN layer 23 a, respectively. As a result, thestructure of a vertical light-emitting diode 2 can be formed.

With reference to FIG. 2, another embodiment can apply additionalprocessing steps on the vertical light-emitting diode 2. After formationof the first electrode 26 and second electrode 27, the second surface IIof the multilayered light-emitting structure 23 can be at leastpartially roughened to form a region comprised of uneven surfaceportions 28. Examples of geometrical shapes for the uneven surfaceportions 28 can include, without limitation, pyramid, cone,half-lenticular shapes and the like. Chemical etching or dry etching canbe used to roughen an area of the planarized surface II of the n-GaNlayer 23 a to form the region of the uneven surface portions 28 apartfrom the area of the second electrode 27. The chemical etching agent beselected from H₂O₂, KOH, TMAH, or any combination thereof, such as amixture of KOH and TMAH. The composition ratio of the etching agent canbe adjusted with the desired etching conditions.

In one embodiment, a two-stage surface treatment can be implemented.After formation of the first electrode 26 and second electrode 27, thestack of layers can be cleaned with an organic solution (such asacetone, methanol, isopropanol or ethanol), and then immersed in achemical etching agent at a temperature between about 20° C. and 200° C.The chemical etching agent can be a solution mixture of KOH and TMAH.After a period of time, the stack of layers can be retrieved and rinsedvia an organic solution. The stack of layers then can perform a secondetching step with the same chemical etching agent as in the first stageand in the same conditions. Subsequently, the stack of layers can berinsed with an organic solution. As shown in FIG. 2, the verticallight-emitting diode 2 formed after the aforementioned two-stage surfacetreatment can include the region of the uneven surface portions 28 thatis apart from the area of the second electrode 27. The region with theuneven surface portions 28 can differ from the pattern of the sapphiresubstrate 21 (e.g., height of the uneven surface portions, density,shape, etc.). With this construction, light exiting the light-emittingdiode can be scattered in a more efficient manner.

In the method described herein, the use of the patterned sapphiresubstrate can significantly reduce the defect density of the stackedepitaxial layers, and improve the properties of light-emitting diode.After the sapphire substrate is removed, the uneven surface of the stackof epitaxial layers initially facing the sapphire substrate (as shown inFIGS. 1C and 1D) can perform a planarization step to prevent theoccurrence of total internal reflection.

In addition, a roughening step can be performed to form the region ofthe uneven surface portions 28 that act to efficiently scatter light outof the light-emitting diode. Examples of shapes for the uneven surfaceportions 28 can include, without limitation, pyramids, cones, and/orhalf lenticular shapes. Depending on the size and number of the unevensurface portions, the emission efficiency of light-emitting diode can beincreased by at least 5% to 10%. In other embodiments, the unevensurface portions 28 may also differ from that shown in FIG. 3 toincrease the emission efficiency of the light-emitting diode.

Realizations in accordance with the present invention therefore havebeen described only in the context of particular embodiments. Theseembodiments are meant to be illustrative and not limiting. Manyvariations, modifications, additions, and improvements are possible.Accordingly, plural instances may be provided for components describedherein as a single instance. Structures and functionality presented asdiscrete components in the exemplary configurations may be implementedas a combined structure or component. These and other variations,modifications, additions, and improvements may fall within the scope ofthe invention as defined in the claims that follow.

1. A method for fabricating a vertical light-emitting diode comprising:forming a stack including a plurality of epitaxial layers on a patternedfirst sapphire substrate, wherein the stack has a first surface incontact with the patterned first substrate, the first surface includinga pattern corresponding to a pattern provided on the first substrate;placing a second substrate on the stack; removing the first substrate toexpose the first surface; planarizing the first surface to remove thepattern and form a planarized second surface of the stack; and forming afirst electrode in contact with a side of the second substrate that isopposite to the stack, and a second electrode in contact with the secondsurface.
 2. The method of claim 1, wherein the stack comprises any of aAl_(x)In_(y)Ga_(1-x-y)N (0≦x≦1, 0≦y≦1) semiconductor material and aoxide semiconductor material.
 3. The method of claim 1, wherein thesecond substrate is placed on the stack of the epitaxial layers by waferbonding.
 4. The method of claim 1, wherein the first sapphire substrateis removed by laser lift-off, dry etching, wet etching or polishing. 5.The method of claim 1, wherein the step of planarizing the first surfaceis performed by applying selective wet etching.
 6. The method of claim5, wherein the selective wet etching includes an etching agentcomprising an acidic solution or an alkaline solution, the acidicsolution being a solution of an acid or a mixture of acids, and thealkaline solution being a solution of a base or a mixture of baseagents.
 7. The method of claim 6, wherein the acidic solution includessulfuric acid (H₂SO₄), phosphoric acid (H₃PO₄), nitric acid (HNO₃),nitrous acid (HNO₂), phosphorous acid (H₃PO₃), hydrochloric acid (HCl),acetic acid (CH₃COOH), carbonic acid (H₂CO₃), boric acid (H₂BO₃), formicacid (HCOOH), iodic acid (HIO₃), oxalic acid (H₂C₂O₄), hydrofluoric acid(HF), hydrosulfuric acid (H₂S), sulfurous acid (H₂SO₃), fluorosulfonicacid (HSO₃F), alkylsulfonic acid (RSO₃F, R═C_(n)H_(2n+1)), or anymixture thereof.
 8. The method of claim 6, wherein the alkaline solutionincludes sodium hydroxide (NaOH), potassium hydroxide (KOH), calciumhydroxide (Ca(OH)₂), tetramethylammonium hydroxide (TMAH), ammoniumhydroxide (NH₄OH), sodium carbonate (Na₂CO₃), sodium hydrogen carbonate(NaHCO₃), potassium carbonate (K₂CO₃), barium hydroxide (Ba(OH)₂), orany mixture thereof.
 9. The method of claim 1, wherein the step ofplanarizing the first sapphire surface is performed by polishing. 10.The method of claim 1, further comprising roughening the second surfaceafter the first and the second electrodes are formed.
 11. The method ofclaim 10, wherein the step of roughening the second surface is performedby chemical etching or dry etching.
 12. The method of claim 11, whereinthe chemical etching uses an etching agent selected from an acid or abase, the acid including sulfuric acid (H₂SO₄), phosphoric acid (H₃PO₄),nitric acid (HNO₃), nitrous acid (HNO₂), phosphorous acid (H₃PO₃),hydrochloric acid (HCl), acetic acid (CH₃COOH), carbonic acid (H₂CO₃),boric acid (H₂BO₃), formic acid (HCOOH), iodic acid (HIO₃), oxalic acid(H₂C₂O₄), hydrofluoric acid (HF), hydrosulfuric acid (H₂S), sulfurousacid (H₂SO₃), fluorosulfonic acid (HSO₃F), alkylsulfonic acid (RSO₃F,R═C_(n)H_(2n+1)) or any mixture thereof, and the base including sodiumhydroxide (NaOH), potassium hydroxide (KOH), calcium hydroxide(Ca(OH)₂), tetramethylammonium hydroxide (TMAH), ammonium hydroxide(NH₄OH), sodium carbonate (Na₂CO₃), sodium hydrogen carbonate (NaHCO₃),potassium carbonate (K₂CO₃), barium hydroxide (Ba(OH)₂) or any mixturethereof.
 13. The method of claim 12, wherein the etching agent includesH₂O₂, KOH, TMAH or any combination thereof.
 14. A method for fabricatinga vertical light-emitting diode comprising: forming a stack including aplurality of epitaxial layers on a patterned first sapphire substrate,wherein the stack has a first surface in contact with the patternedfirst substrate, the first surface including a pattern corresponding toa pattern provided on the first substrate; placing a second substrate onthe stack; removing the first substrate to expose the first surface;planarizing the first surface to remove the pattern and form aplanarized second surface of the stack; and forming a first electrode incontact with a side of the second substrate that is opposite to thestack, and forming a second electrode in contact with the second surfaceand; applying a two-stage surface treatment to roughen at leastpartially the second surface to form a region comprised of unevensurface portions having a geometrical shape, wherein the one stage oftwo-stage surface treatment comprises: cleaning the surface with anorganic solution, and roughening the surface by using a chemical etchingagent.
 15. The method of claim 14, wherein the etching agent is selectedfrom H₂O₂, KOH, TMAH or any combination thereof.
 16. The method of claim14, wherein the organic solution is selected from acetone, methanol,isopropanol, ethanol or any combination thereof.
 17. The method of claim14, wherein the geometrical shape comprises pyramids, cones, and/or halflenticular shapes.